Group-III nitride semiconductors including gallium nitride (GaN), aluminum nitride (AlN), or aluminum gallium nitride (AlGaN), for example, have large dielectric breakdown field and carrier velocity, thus application thereof to high-power electronic devices or optical devices is expected. However, there is a problem that output characteristics are limited by self-generated heat in a high-power operation.
Accordingly, an attempt to solve the problem described above has been made by forming a nitride semiconductor layer on diamond to manufacture a field effect transistor (FET). Since diamond has extremely high heat conductivity, a field-effect transistor provided on diamond is capable of a high-power operation in principle which cannot be achieved on the other substrate in principle.
However, it is hard to grow a single-crystalline nitride semiconductor layer directly on diamond in a conventional technique (Non-Patent Document 1). A crack occurs even in a polycrystalline nitride semiconductor layer when a film thickness thereof is large, that is equal to or larger than 1000 nm. Since the polycrystalline nitride layer with the crack has extremely low electrical conductivity, a drain current of the field effect transistor manufactured by growing the nitride semiconductor layer directly on diamond is low, so that high output power density cannot be obtained.
Accordingly, in Patent Document 1, a thin film made of Si is laminated on a diamond layer to make a relaxation layer, and a nitride semiconductor layer is formed on the relaxation layer, thereby reducing an occurrence of crack.
Patent Document 2 discloses that diamond is grown on a nitride semiconductor substrate. The nitride semiconductor substrate includes a plurality of epitaxial layers (single-crystalline nitride layer) on an upper surface side and includes a device layer on the epitaxial layers. Diamond is formed in a hole formed on a bottom surface side. In this case, diamond is grown on the nitride, thus diamond microparticles (seeds) are previously dispersed on the nitride.